Neutron detectors are a component technology supporting neutron beam application technologies. With the progress of neutron application technologies in academic research fields such as neutron diffraction, in nondestructive inspection fields, or in security fields such as cargo inspection, neutron detectors with higher performance are desired.
Main performances demanded of the neutron detector are a detection efficiency for neutrons, the count rate of neutrons, and the ability to discriminate between neutrons and gamma rays (may hereinafter be referred to as n/γ discrimination ability). The detection efficiency means the ratio of the number of radiations counted by the detector to the number of radiations emitted from a radiation source and entered into the detector. The count rate means the number of radiations counted per unit time. γ rays are generated when neutrons hit an element—contained in a constituent member of a detection system for detecting neutrons, or in an object to be tested—such as Fe (iron), Pb (lead), Cd (cadmium), C (carbon) or N (nitrogen). If the ability to discriminate between neutron radiation and γ rays is low, a false signal due to gamma ray which does not reflect the interaction between neutrons and the object to be tested contaminate the true neutron signals, and a so-called background noise increases.
Neutrons have a high power to pass through a substance without doing any interaction in the substance. Therefore, a nuclear reaction for promptly converting neutrons into charged particles having energy is generally utilized to detect the neutron radiation. For example, a helium-3 (3He) detector which detects neutrons by utilization of protons or tritons generated by a nuclear reaction between 3He and neutrons has so far been known. This detector has high detection efficiency and excellent n/γ discrimination ability, but has posed the problem of a limited count rate. Moreover, 3He is an expensive substance and its resources are limited.
Recently, the development of a detector using a neutron scintillator, instead of the above-mentioned 3He gas process, has been underway in an attempt to produce an inexpensive and upsized detector. The neutron scintillator refers to a substance which, when hit by neutrons, absorbs the neutrons to emit fluorescence. The aforementioned various performances of a neutron detector using this scintillator depend on a substance constituting the scintillator. If the scintillator is composed of a light element which minimally interacts with γ rays, for example, the aforementioned background noise due to γ rays is reduced. The light yield from the scintillator affects the ratio between a pulse signal attributed to luminescence produced upon detection of neutrons and a pulse signal associated with an electrical noise, namely, S/N ratio. The content of 6Li isotope or the like, which captures neutrons with high efficiency, influences the detection efficiency. The decay time of fluorescence influences the count rate.
LiF/ZnS has been used as a neutron scintillator having a relatively high neutron detection efficiency and excellent n/γ discrimination ability (see Non-Patent Document 1). Since the LiF/ZnS is opaque, however, an increase in the thickness of the scintillator has made it impossible to take out scintillation light efficiently. Thus, the LiF/ZnS has been limited in the improvement of neutron detection efficiency.
In view of such problems, a proposal has been made for a neutron scintillator comprising a eutectic composed of europium-containing calcium fluoride crystals and lithium fluoride crystals (see Non-Patent Document 2). This neutron scintillator composed of the eutectic is translucent, and enables scintillation light to be collected with high efficiency. Thus, this neutron scintillator can achieve a very high neutron detection efficiency. According to studies by the inventors of the present invention, however, the eutectic has been poor in n/γ discrimination ability, and still has left room for improvement. Furthermore, the eutectic containing calcium fluoride crystals has posed the problems that when used as a scintillator, it does not provide a sufficient light yield, and it is insufficient for discriminating between a pulse signal due to luminescence generated upon detection of neutrons and a pulse signal caused by an electrical noise.